The invention relates to substrate processing, and more particularly, to the handling of substrates in and around "processing islands", which may include just a processing chamber, a processing chamber with load locks, or a set of processing chambers with or without load locks.
Most semiconductor processes are automated. For example, automatic temperature controllers are used to heat a substrate to a predetermined temperature for a predetermined period of time as dictated by a process-control computer. Most processes are run by such a computer according to a "recipe" input by an operator.
One level of automation involves the loading and unloading of substrates. A cassette of typically 20 to 25 substrates is often used for such operations. Some process equipment employs a buffer storage capability for cassettes to increase efficiency by always having fresh substrates available for processing and a place to unload processed substrates. Automatic guided vehicles ("AGV"s) may also be used in this type of automation. AGVs travel along the aisles of a production line and dispense cassettes of substrates as required. This technique is useful for production lines in which the equipment is organized in rows. Besides AGVs, overhead rails may also be employed to transport cassettes of substrates.
Another level of automation employs sequential performance of specific processing steps. Such processing steps may involve separate processing machines or stations in an assembly line-like setting. "Clustering" combines two or more process steps in a single unit by using more than one process chamber surrounding a central loading chamber having a loading robot. Two or more sequential processes performed in a cluster system are referred to as "integrated processes". For example, a substrate may be placed in one process chamber for etching, a second process chamber for cleaning, and a third process chamber for metal deposition. Clustering also allows improved throughput by parallel processing multiple wafers in a single process step.
An improved cluster system is shown in FIG. 1. This system 20 includes a vacuum robot 22 in a chamber surrounded by processing chambers 24A-24D, load lock cooling chambers 26A and 26B, and heating chamber 28. The chambers 26A, 26B and 28 each contain a substrate cassette for holding a plurality of substrates. Substrates may be delivered to or removed from the chambers 26A and 26B. Specifically, the substrates may be exchanged for substrates in a plurality of cassettes 30 by an atmospheric exchange system 32.
The glass substrates can have dimensions, for example, of 550 mm by 650 mm. The trend is toward even larger substrate sizes, such as 650 mm by 830 mm and larger, to allow more displays to be formed on the substrate or to allow larger displays to be produced. The larger sizes place even greater demands on the capabilities of the processing systems.
To facilitate clustering, robotic systems have been used to transfer substrates from one processing station to another. Because of the high cost of robotics, including their associated controls and programming, one robot is often used to service a number of machines. Although such use is flexible, allowing accommodations to different or changing physical environments or processes, the robots are still expensive to acquire. They represent a single failure point which can affect an entire processing system. Although robots can be used to service a number of stations, they must service those stations sequentially, thus limiting the efficient use of the stations.
Yet another level of automation is providing for two or more sequential process steps in a single process chamber. This desirably eliminates an unloading and loading step, increasing cleanliness and throughput. For vacuum processes, time and cleanliness are favorably affected when more than one process can be performed with only one pump-down of a chamber.
Drawbacks of clustering include a greater reliance on interlocks, electronics, and software than is required for individual tools. Downtime also affects a larger part of the production capacity than is true for individual tools. In some cases, preferred cluster modules can only be supplied by different vendors, leading to difficulties in compatibility.
Even worse, the addition of a cluster tool in a factory requires a larger incremental factory size due to the floor space required for several process chambers. In other words, the minimum incremental factory size is larger for a cluster tool than for a single process chamber. Accordingly, cluster tools are currently arranged in a "job shop" configuration where all the process chambers perform the same or similar processes. Such a configuration is acceptable from a cost standpoint, but is practical only in large increments of factory capacity. A "mini-fab" factory configuration is not suitable for cluster tools because of the large incremental capacity of multi-chamber cluster tools.
A further drawback of cluster tools is a potentially inefficient matching of "TACT" times. The "TACT" time is the total actual cycle time and refers to the time period between the introduction of the substrate into the process system and its subsequent removal from the system. The TACT times of the various pieces of process equipment must substantially match in order for the factory to operate in an efficient serial sequence. As the mismatch in TACT times increases, the number of substrates required to keep the factory running likewise increases, along with the cost of operation. TACT times of cluster tools are less cost-effectively matched to one another, because cluster tools are purchased in larger per-system capacity increments.